There has been used a coating composition comprising an aluminum flake pigment having flake thicknesses in the range of 0.1-1 .mu.m and an average particle size of 5-60 .mu.m, which has been treated by grinding in a ball mill together with a fatty acid, for providing the coating film with a brilliant appearance.
The aluminum flakes to be blended in the metallic paints as a brightening pigment are classified according to their properties into a leafing type and a non-leafing type. The leafing type aluminum flake pigment is available usually in a state in which the flakes are treated by grinding them together with stearic acid. Due to a reduced interfacial tension, this pigment is liable to float up onto an upper layer of the metallic coating film when being coated and to form a structured coating layer in which the flakes align over one another in parallel to the substrate, resulting in an appearance of successive aluminum layers. However, the aluminum flake pigment which has floated up on the top face of the upper layer is not fixed in the coating film after the coating film cures, so that the flakes may easily be peeled off by touch. On the other hand, when a clear coating is applied onto an uncured coating film in a wet-on-wet technique, the orientation of the aluminum flakes is disturbed and the appearance deteriorates. When a clear coating is applied onto the metallic coating film after the film has been heat-cured, a reliable adhesion of the coated film and the clear coating layer will not be guaranteed.
For these reasons, the practice has been to use principally the latter, i.e. non-leafing type aluminum flakes in industrial paints.
In conventional metallic paints containing only non-leafing type aluminum flakes, these flakes are usually contained therein after having been ground together with oleic acid, whereby a problem is brought about in that the aluminum flakes added as a brightening pigment have a larger thickness and may be dispersed within the metallic coating layer upon its application on a substrate, so that each aluminum flake is visually recognizable as a discreet particle and a plated metal-like appearance of the coating is not attained, though the adhesion of the metallic coating to the layer of the clear coating is better.
As a coating composition in which the problem of appearance of a metallic coating is improved, there has been proposed in Japanese Patent Kokai Sho-53-9836 A a coating composition characterized in that it comprises a volatile carrier, fine metal particles and a binding material, wherein the fine metal particles consist of metal platelets having thicknesses not greater than 1,000 .ANG. shaped generally irregularly and present in such an amount that the weight ratio of the binding material to the metal platelets is in the range from 0.05/1 to 10/1. This metallic coating composition is, however, inferior in hiding power for hiding sand scratches on the substrate and does not permit the formation of a coating film exhibiting a superior continuous plated metal-like appearance in a stable manner.
An object of the present invention is to provide a method for forming a coating film, in which a coating film having no problems in its properties and which is superior not only in its hiding power for hiding sand scratches on the substrate but also in a continuous plated metal-like appearance can be built up in a stable manner.
Another object of the present invention is to provide a coating composition which can be applied by the method for forming a coating film as above and form a coating film having satisfactory film properties and which is superior not only in its hiding power for hiding sand scratches on the substrate but also in that a continuous plated metal-like appearance can be built up in a stable manner.